As is well known a GaAs MESFET constitutes two parallel ohmic contacts and a gate (electrode or barrier contact) centered between these ohmic contacts positioned on a GaAs substrate which is highly doped and which itself is positioned on a semi-insulating base or substrate.
It has been shown that the performance of this device improves in terms of output power and increased operational frequency when the active layer region depleted of free electrons in the section beneath the gate and the gaps between the gate and the ohmic contacts has a greater depletion depth under the gate as compared with the gaps between the gate and the ohmic contacts. This effect is clearly in the absence of an applied voltage to the gate.
This improved performance is due to the fact that in the situation described above, the channel region which is modulated by the gate voltage is found to be better defined and the input resistance, that is the resistance of the section of active layer from the source to gate, for equal thickness of the undepleted zone in the section from the source to drain, is found to be smaller (e.g. IEEE Transaction on Electron Devices VRED-27 No. 2 February 1980).
Confirmation of the characteristics mentioned above has been obtained by realising-by means of the preceding technique a transistor with a recessed gate with respect to the surrounding area. (e.q. the IEEE pubblication already cited).
However a process for realising a recessed barrier electrode presents numerous difficulties. In particular the structure cannot be obtained when the barrier electrode is defined, that is deliniated, simultaneously with the two ohmic contacts which are parallel to it by means of a self-aligning process in which, as is well known, the barrier electrode metallization is uniformly deposited on the active layer surface and then deliniated by chemical etch.